1. Technical Field
This invention relates generally to the batch processing of miniature electronic circuit components, including passive, two-terminal, ceramic capacitors, resistors, inductors, and the like. More particularly, it concerns a test plate for holding such components or other type of device under test (DUT) as part of the batch processing for purposes of parametric testing.
2. Description of Related Art
The tiny size of electronic circuit components of interest herein complicates processing. Typically fabricated in parallelepiped shapes having dimensions as small as 0.020xe2x80x3 by 0.010xe2x80x3 by 0.010,xe2x80x3 more or less, these difficult-to-handle components require appropriate equipment and precision handling techniques. What is sometimes referred to as a xe2x80x9ccarrier platexe2x80x9d holds many hundreds of the components upright in spaced-apart positions as the ends of each component are coated with a conductive material to produce electrical terminals. After adding terminals, a xe2x80x9ctest platexe2x80x9d holds the large batch of components for movement past a contactor assembly of a testing system for parametric testing purposes and eventual sorting. Thoughtful design of each of these components promotes efficient processing. Reference may be made to U.S. Pat. Nos. 6,204,464; 6,294,747; 6,194,679; 6,069,480; 4,395,184; and 4,669,416 for examples of some prior art component handling systems and testing techniques.
The test plate is of particular interest. Mechanically, the test plate must hold the DUTs securely enough as they move past the contactor assembly so that they are presented to the contactor assembly in a repeatable, mechanically stable position. Electrically, the test plate must not degrade test results. But the mechanical and electrical functions are conflicting. Various forms of grease, grime, dirt, dust and other electrically conductive material on the test plate and/or on the DUTs provide unwanted conductive paths (i.e., stray impedances) to the DUT terminals. The stray impedances can render test results inaccurate. Thus, manufacturers engaged in batch processing of miniature electronic circuit components seek improvement in test plate design in that respect and so a need exists for a better test plate.
This invention addresses the concerns outlined above by providing a multilayer test plate. It may be used in testing any of various passive components, including capacitors, resistors, multilayer inductors, inductor beads, varistors, thermistors, fuses, sensors, actuators, and the like. The multilayer test plate has at least two layers, one conductive and one nonconductive. The nonconductive layer holds the DUTs while the conductive layer functions as a guard layer that enables the measurement system to eliminate, or at least significantly reduce the effects of stray impedances. The test plate can be configured as a direct replacement for existing test plates, and one embodiment even includes an additional guard layer that includes a pattern of rings (guard tracks) between rings of DUT-engaging holes in the two nonconductive layers.
To paraphrase some of the more precise language appearing in the claims, the invention provides a test plate in the form of a DUT-holding plate having a rotational axis and at least two layers centered on the rotational axis. A nonconductive layer of the two layers is composed of an electrically nonconductive material (e.g., epoxy printed circuit board material) that defines a plurality of DUT-engaging holes. A conductive layer of the two layers is composed of an electrically conductive material (e.g., copper) that defines a plurality of oversized holes such that each of the oversized holes is in alignment with a respective one of the DUT-engaging holes. The oversized holes have a size larger than the DUT-engaging holes in order to avoid having the conductive layer contact a DUT held by the test plate. That arrangement enables use of the conductive layer as a guard layer held at a guard potential for electrical testing purposes. It can be held at a desired guard potential and thereby eliminate, or at least significantly reduce, the effect of stray impedances on test results.
One embodiment of the invention includes a second conductive layer on an opposite side of the nonconductive layer. The second conductive layer can be used as a second guard layer held at a second guard potential that is the same or different from the guard potential at which the first conductive layer is held. The second conductive layer may include oversized holes and/or one or more radially spaced-apart conductive rings (i.e., guard tracks) disposed intermediate radially spaced-apart rings of DUT-engaging holes in the nonconductive layers. In addition, the invention can be readily fabricated by etching conductive patterns on double-sided, copper-clad, printed circuit board material to result in the first nonconductive layer being sandwiched in between the two conductive layers. Additional conductive and nonconductive layers can be added thereafter.
Thus, the invention provides a multilayer test plate that improves upon and better balances its mechanical and electrical functions. It includes at least two layers, at least one of which is a conductive guard layer. It may include one or more additional guard layers and/or nonconductive layers. It can be fabricated using printed circuit board techniques, and it can be configured as a direct replacement for existing test plates. The following illustrative drawings and detailed description make the foregoing and other objects, features, and advantages of the invention more apparent.